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A 
ccc "PSYCHOLOGICAL REVIEW PUBLICATIONS Rn eere oat 158 


Psychological Monographs 


EDITED BY 


SHEPHERD I. FRANZ, University or CattrorniA, So. Br. 
HOWARD C. WARREN, Princeton University (Review) 
JOHN B. WATSON, New York, N. Y. (J. of Exp. Psych.) 
MADISON BENTLEY, University or Itirno1s (/ndex), and 
S. W. FERNBERGER, University or PENNsyLvania (Bulletin) 


The Energy Value of the Minimum 
Visible Chromatic and Achromatic 


For. Different Wave-Lengths 
of the Spectrum 


By 


MARGARET M. ‘MONROE 


Bryn Mawr COLLEGE 


PSYCHOLOGICAL REVIEW COMPANY 


PRENCH TO NEN se 
AND ALBANY, N. Y. 


Acents: G. E. STECHERT & CO., Lonpon (2 Star Yard, Carey St., W. C.) 
Leripzic (Hospital St., 10); Paris (76, rue de Rennes) 


Digitized by the Internet Archive 
in 2022 with funding from 
Princeton Theological Seminary Library 


https://archive.org/details/energyvalueofminOOmonr 


PREFACE 


In the work reported in this dissertation three important deter- 
minations have been made at seven points in the spectrum: the 
minimum visible achromatic and chromatic and the photochromatic 
interval. or the first time these determinations have been made 
by direct measurement in absolute energy terms. The inves- 
tigation is one of a series of studies the object of which has 
been primarily to lay a foundation for the exact measurement of 
human responses in terms that are quantitative or numerically 
comparable. Such work has been possible only within the last 
few years. The importance of the study as a foundation for the 
more scientific phases of medical work on the eye should also be 
noted. Further work on this and other applications of functional 
testing to the study and diagnosis of diseases of the eye will be 
carried on by the author in a research position in the Graduate 
Medical School of the University of Pennsylvania. 

The subject of this dissertation was suggested by Professor 
C. E. Ferree and Dr. G. Rand of the Department of Psychology 
of Bryn Mawr College and the dissertation was prepared under 
their direction. The writer wishes to acknowledge her deep 
indebtedness to Professor Ferree and Dr. Rand for their careful 
supervision and constant help during the work on the dissertation 
and for their great kindness and support during the whole of her 
graduate course. She takes pleasure also in expressing her 
appreciation to Dr. Rand for her kindness in making the energy 
measurements given in Table V. Thanks are likewise due to 
Dr. Luther C. Peter, Associate Professor of Ophthalmology, 
Philadelphia Polyclinic and College for Graduates in Medicine, 
University of Pennsylvania, for his courtesy in sending patients 
for the section on pathological cases. 


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CONTENTS 


PAGE 
IRL TRODU GLUON GE aU MARE: ce co digas a Rocka a/6 oo sacar TORR ae 1 
Mined ESTORICAIS UM MARY afrika sinter oe bits ca's so Lae 5 
PILE PARATUS(AND EROCEDURE. oF Gy 45.0.0 cs,08 ose a od oe eae 15 
EAM SELEY OUT CE OLE LACIE OE ON aay cle cif alee sn mists ee oS 
Sere Larter S POCUP ORCI PEN ae a Tre Ske sere aay Wis ais a Se 16 
‘C. Apparatus for Presenting the Light to the Eye.... 19 
D. Means of Reducing the Intensity of Light........ Os 
Wel nepkilterse eset enw. ra weeeeitels sue tn 22 
UAL HE) CVECEOT OCS DO TSCRu te eon el crt Ss oye tee 23 
Die Dea edecaie. cakes sips cit choi Daas amt 3 23 
rl ne adtometriccA pparains git os as nga hie 24 
FAT heik nergy COsUremnent san oy cried oan adie sashes 24 
Cruel he ethos wt ODServaHon. ix eis «ching » Vises 27 
LV. STATEMENT AND DISCUSSION OF RESULTS. . 8... f. 062.6) 28 
FA CRvOMae LIM ESHOLAS A). es ave e so OEE) eee 28 
SC UITOIMCTICML LT ESHIOLIGS oe oe x ind Ca eae elven 34 
C. The Photochromatic Interval .05.0....0000 0 eee ee 43 
D. Comparison with Previous Determinations of the 
Le SHOLGs CASHEL Ctr en wd oe ord eunane Vaio de 
meter” IVE SPATE be ESE DIC cc Let kis voces Wecm,e een: ale Soke 48 
PeMIOL OL OCACOL, CSG 5 Nyc eer lei Ae nla Re i8ed (edgy Mx, 54 


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I 
INTRODUCTION 


By the minimum visible is meant the least amount of light 
radiation to which the eye is capable of giving a visual response. 
Obviously the term may be used, broadly, to mean the least amount 
of light of any composition whatever to which the eye gives a just 
noticeable response; or, narrowly, as the least amount of the 
wave-length to which the eye is the most sensitive. Apparently 
the purpose of a recent group of writers has been to favor the 
narrower usage of the term although it has been variously applied 
by different members of the group to determinations made with 
the light of the stars, the light of a tungsten lamp, and to wave- 
lengths in the mid-region of the spectrum. The broader usage, 
which is the older, is more compatible with the purpose of this 
investigation and will be conformed to in the statement and dis- 
cussion of results. Used in this sense, the minimum visible is 
synonymous with the absolute threshold or limen. 

Numerous points of interest may attach to the determination 
of the minimum visible. 

(1) There may be a scientific curiosity to know the least 
amount of energy to which the eye is capable of giving a response 
and to compare this with the least amount to which the ear or 
some other sense organ gives a noticeable reaction. Wien, for 
example, was led to attempt a determination of the minimum 
visible by his previous work on the minimum audible. A natural 
extension of this interest is a comparison of the sensitivity of the 
eye with that of the physical instruments which respond to light. 
Coblentz has found, for example, that it has, roughly speaking, 
300,000 times as great a sensitivity to light radiations as the most 
improved type of thermopile. 

(2) In the evolution of the sense organs, many complex char- 
acteristics and adaptations have developed presumably in the 


interests of functional efficiency. The eye, for example, has 
1 


2 MARGARET M. MONROE 


developed to a high degree a selectiveness of response to light 
radiations. Obviously in adapting light to the service of the eye 
in problems of illumination and in making a correct use of the eye 
in the many scientific and technical ways in which it is employed, 
it is highly important, therefore, to know minutely this selective- 
ness of response both as to kind and amount. It is also interest- 
ing from explanatory and technical points of view to be able to 
compare the sensory with other known and better understood 
types of response such as the photoelectric, the photochemical, 
and the thermoelectric reactions, the action of light on selenium, 
etc. For example, explanatory theories of the eye’s response 
have already been developed in terms of two of these types of 
reaction—the photochemical and photoelectric—and all of them, 
including the sensory reaction, have been utilized at various times 
for rating light intensities for scientific and technical purposes. 
(3) It is generally recognized that the most sensitive means 
of detecting the eye’s abnormality due to natural causes or its 
subnormality due to pathological conditions and processes is in 
terms of its lack or loss of sensitivity or power to give response, 
relative and absolute. There are many practical applications of 
this principle. For example, (a) one of the ways in which 
ocular fitness for vocational purposes is rated is a determination 
of the light and color sense. Eyes vary a great deal in light 
sensitivity, particularly in their range of sensitivity to light intensi- 
ties. Many, for example, who are able to qualify for work at 
medium and high intensities of illumination are disqualified for 
vocations requiring keen power and quickness of vision at low 
illuminations. Others lack in color sensitivity, varying from a 
complete absence of power to sense one or more colors to slight 
deficiencies which disqualify only for work which requires special 
powers such as great keenness of discrimination, speed of dis- 
crimination, etc. And (b) the most pronounced and earliest 
manifestation of pathological conditions of the sensory mechan- 
ism is the loss of light and color sensitivity. This varies from 
a slight deficiency to a total loss of function, depending upon the 
severity and stage of advancement of the pathological condition. 
The application of the testing of light and color sensitivity to 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 3 


diagnosis, however, is in its infancy, partly because of the lack 
of instruments and methods which are feasible for office and clinic 
work and partly because of a lack of knowledge of norms of 
sensitivity for the healthy eye and of the deviation from these 
norms which are characteristic of disease and which differentiate 
one diseased condition from another. 

Some of the known effects of pathological conditions are a 
diminished light sensitivity, a particular and differential manifes- 
tation of which is a greatly diminished or entire absence of power 
to see at low illumination, absolute and partial loss of color 
sensitivity, changes in the relative sensitivity to the different 
colors, and changes in the interval between the achromatic and 
chromatic thresholds of sensitivity to colored lights—the photo- 
chromatic interval. As has already been stated, but little use has 
been made of the testing of light and color sensitivity of the 
central retina by the medical profession. Their work up to the 
present time has been confined chiefly to the mapping of the fields 
of light and color sensitivity. Even this has been almost pro- 
hibitive because of the care required to obtain an acceptable 
precision of result, and the time consumed in making the 
determinations in a sufficient number of meridians. It is a matter 
of great interest and importance, therefore, to see how far central 
sensitivity testing, which with methods amply sensitive for diag- 
nostic work need consume but little time, can be substituted for 
field taking. As already indicated, field taking is at best a time 
consuming and difficult performance from the standpoint of the 
patient, the physician, and the apparatus and controls required. 

While the medical aspects of the problem have made the strong- 
est appeal to the writer’s personal interests, it has been deemed 
advisable to subordinate them at this time to an endeavor to build 
a sounder groundwork on which to rest the applications. It is 
important in every applied field that there should be standards of 
reference as to apparatus, methods, and results against which the 
deviations made in the interests of convenience and feasibility can 
be checked and evaluated and from which fresh starts can be made. 

There are two important aspects of the testing of retinal 
sensitivity: the testing of the absolute sensitivity to the different 


4 MARGARET M. MONROE 


wave-lengths of light and a determination of the comparative or 
relative sensitivity to these wave-lengths. Differences in both 
of these regards are fundamental in all of the fields in which 
sensitivity testing and its results may be applied. To serve as 
a standard of reference it is obvious that absolute sensitivity 
should be expressed in absolute terms both with regard to the 
composition of the stimulus and to its intensity, i.e., for the eye, 
in terms of wave-length and energy value of light. It is equally 
obvious that if the sensitivities to the different wave-lengths are 
to be compared, the intensity values of the stimulus of which 
sensitivity is taken as the reciprocal must be expressed in terms 
that are numerically comparable, i.e., in terms of the physical 
intensity or energy value of the lights employed. Norms either 
of chromatic or achromatic sensitivity have not as yet been 
determined in absolute terms. Fragmentary attempts have been 
made on the basis of results obtained from a few observers to 
express achromatic sensitivity in relative terms. The investiga- 
tion of chromatic sensitivity, however, has not progressed even 
this far. The work of establishing norms of absolute and rela- 
tive sensitivity for both of these types of response will involve 
expenditure of a great deal of time and effort and the cooperation 
of many people. Even to contemplate it seems overambitious at. 
this time. However, every work must start from small begin- 
nings. It has been the purpose of this study to make such a 
beginning of the study of the thresholds of chromatic and 
achromatic sensitivity. 

The following determinations have been made at seven repre- 
sentative points in the spectrum: 


(1) The achromatic threshold. — 
(2) The chromatic threshold. 
(3) The photochromatic interval. 


Twenty-one observers have been used in making these deter- 
minations and in every case a direct energy measurement was made 
of the light used to stimulate the eye. In addition, determinations 
were made in a limited number of pathological cases. 


II 
HISTORICAL SUMMARY 


As early as 1888 Hermann Ebert (1) attempted to ascertain 
numerically the relative achromatic sensitivity of the eye to wave- 
length. Two lines of evidence had led him to suppose that the 
eye might have a maximum of sensitivity to green. In his con- 
clusions the emphasis is on the position of this maximum rather 
than on the details of the threshold visibility curve. The first 
probleni, his more immediate incentive, was the explanation of 
the striking simplicity of the spectra of the gaseous nebulae which 
in most cases consist merely of lines in the green and the blue- 
green, A 500,495 and 480 wu. Two theories of this phenomenon 
had already been advanced: first, that only these wave-lengths 
are emitted, and second, that there is selective absorption in intra- 
nebular space. Ebert thought it more probable that the cause lies 
within the observer. That is, if the eye should prove to be most 
sensitive to green, it would be reasonable to expect that in a weak 
spectrum, such as that given by the gases, only the green would 
be visible. His second, more general motive, was the reopening 
for discussion of the whole psychophysical question, a subject 
made pertinent by the work of Weber (2) and Stenger (3) on 
Draper’s law during the previous year. In discussing the results 
obtained in this work, Weber had assumed a direct proportionality 
between the responses of the eye and the energy of the light wave. 
Stenger had pointed out the incorrectness of this assumption but 
his discussion of the matter had been slight enough to warrant a 
more complete investigation by Ebert. 

Draper’s law refers to the order in which the wave-lengths of 
the visible spectrum emitted by incandescent solids reach the 
threshold of sensation with increase of temperature. Draper had 
stated that all metals begin to glow at the same temperature, 
about 525° C., and that the development of the light emission 
runs the following definite course: at 525° the light emitted gives 

5 


6 MARGARET M. MONROE 


a spectrum which reaches from line B to line b; at 625° from 
B to F; at 718° from B to G; and at 1165° it gives a spectrum 
approximating that of the sun in extent. According to Draper, 
then, the spectrum of a glowing solid whose temperature is gradu- 
ally increased develops practically in one direction only—from 
red to blue. . 

In the course of some experiments on the relation between the 
brightness and consumption of energy in carbon lamps, Weber 
had noted some entirely unexpected phenomena which caused him 
to doubt the correctness of this law. Watching the development 
of the light emission of the carbon filament with increase of 
temperature in a dark room, he noted that the red glow was not 
the first light visible, but that another light appeared and under- 
went an entire series of changes before there was any sign of 
color. This light he designated as “ gespenstergraue’’ or 
‘“dusternebelgraue.”’ At first it was very unstable, although 
whether its rapid appearances and disappearances were due to 
change of intensity of the light because of fluctuation in the 
temperature of the filament, or whether they are due to an eye 
phenomenon, he did not know. With a slight increase of 
intensity the light became a little less variable although it retained 
its “ dustergraue”’ quality. With greater increase the gray light- 
ened, and the coloration gradually changed through an ashgray 
to a definite “ gelblichgraue.’’ By the time the red glow was 
visible, the light had become fixed, losing the flickering quality 
which it had had throughout the previous changes. 

A prismatic analysis of this first gray light with the complete 
spectroscope was not possible because of the weak intensity, but 
Weber was able to observe the changes through a direct vision 
prism, and, at slightly higher intensities, through a grating. The 
spectrum of the ‘“ dusternebelgraue”’ light, when first strong 
enough to be seen through the prism, consisted of a homogeneous 
gray strip which occupied the position where at higher intensities 
yellow and yellow-green appeared. As the temperature was 
increased this strip broadened and brightened. When the tem- 
perature had reached a degree such that with the naked eye the 
light looked “ gelblichgraue,” its spectrum was seen as a broad. 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 7 


band, yellowish-gray in the center and shading to a weak gray on 
either side. When, viewed by the naked eye, the light was red- 
dish, there appeared in the spectrum at one side of the gray strip 
a small “ feueroth ”’ space, and almost simultaneously on the other 
side a gray-green strip. As the light became whiter with increase 
of temperature the spectrum continued to develop until finally the 
entire spectrum was present. This description of the light emis- 
sion of a glowing solid was verified by Stenger, who repeated 
Weber’s experiment. 

It is in his conclusion that Weber makes the assumption that 
visibility is proportional to energy, an error which, as was stated 
above, was pointed out by Stenger. Weber states his conclusions 
as follows: 


~“Das Spectrum des glithenden Kohlenfadens wachst also bei steigender 
Temperatur nicht einseitig, in der Richtung vom Roth nach dem Violett, 
sondern entwickelt sich, von einem schmalen Streifen ausgehend, genau von 
seiner Mitte aus, gleichmassig nach beiden Seiten. Die dem Auge zuerst 
erscheinende, den Ausgangspunkt der Spectrumsentwickelung bildende Strah- 
lung ist dieselbe Strahlung, die im vollstandig entwickelten sichtbaren 
Spectrum dem Auge mit der grodssten Helligkeit leuchtet und in dem 
schwarzen Flachen der Thermosdule und des Bolometers die maximale 
Energie entwickelt. 

“Daraus ist wohl der Schluss zu ziehen, dass diese Strahlung mittlerer 
Wellenlange deswegen dem Auge am friihesten sichtbar wird. weil sie auch 
schon bei der Temperatur der beginnenden Graugliiht die maximale Energie 
besitzt, infolge dessen ihre lebendige Kraft am friihesten jenen Schwellen- 
werth tbersteigt, welcher vorhanden sein muss, um eine Lichtempfindung zu 
veranlassen, und dass die iibrigen Strahlungen kleinerer und gr6sserer 
Wellenlange dann bei steigender Temperatur der Reihe nach dem Auge 
sichtbar werden, sobald deren lebendige Kraft einen Schwellenwerth 
ahnlicher Grosse tiberstiegen hat.” 


Stenger, referring to Langley’s (4) results on the energy dis- 
tribution of the sun’s spectrum, showed that since the greatest 
energy is in the red, not in the green or yellow-green, the eye 
must be selectively sensitive, and that the maximum of this 
sensitivity must lie in the green. 

It was at this point that Ebert took up the problem. The 
details of his experiment are as follows: The source of light was 
a gas flame which illuminated from behind a screen of oiled 
paper. This screen was assumed to be evenly illuminated and 
was focussed on the slit of the spectroscope by a lens 12 cm. in 


8 MARGARET M. MONROE 


diameter placed at 125 cm. from the slit. The observer, looking 
through the objective slit, saw the face of the prism filled with 
spectral light. The intensity of the light was then reduced until 
it was no longer visible. For each wave-length used, determina- 
tions of the threshold were made in ascending and descending: 
series, and the mean of the two series calculated. The variation 
amounted to between 2 and 3 per cent. His values are for only 
five different wave-lengths and for two observers. The length 
of the adaptation period is not given; Ebert states merely that 
the experiment was performed after “ sufficient ” dark-adaptation. 

The reduction of the light intensity was obtained by a dia- 
phragm, .07 cm. in diameter, placed between the focussing lens 
and the collimator slit, and so mounted that it could be moved 
over the entire distance—125 cm. As this diaphragm is moved 
nearer to the focussing lens, it decreases the diameter of the used 
portion of the lens, thus reducing the intensity of light focussed 
at the slit. If E is the distance of the diaphragm in cm. from 
the slit, and D the diameter of the used portion of the lens, then 


125 X .07 
Da 
E 


The intensity of any part of the spectrum is then proportional 
touD*: 

The distribution of energy in the spectrum of the light source 
used was calculated indirectly by combining the results of Langley 
mentioned above, with those of Meyer (5), who had checked the 
energy distribution of a gas flame, the source used by Ebert, 
against that of the sun. Langley, using a bolometer, had deter- 
mined the energy distribution of the sun’s spectrum in relative 
terms, 1.¢., in terms of galvanometer readings—no absolute values 
are given. Meyer had compared photometrically the spectrum 
of a gas flame with that of the sun, obtaining a series of ratios 
representing the energy distribution of the gas flame relative to 
that of the sun. Ebert, to get the energy distribution of his own 
source, multiplied the values given by Langley by the appropriate 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 9 


ratios as given by Meyer. The results of these calculations are 
shown in Table I, quoted from Ebert. 


TABLE TL 
Helligkeit Gaslicht E 


Mittlere Helligkeit Sonne Sonne E 

Farbe Wellenlange (Meyer) (Langley) Gaslicht 
Oth Irae clea ee 675 pp 4,07 62 252 
Gel Die anette ee hore 590 uy 1,00 45 45 
Grits ee es 530 pp 0,43 28 12 
isrunblauneed. ir oe Ale 500 py 0,43 22 10 
(VEG Geond Baleege oor 470 py 0,23 14 3 


In this table the second column gives the middle wave-length 
of the various spectrum bands used; the third volumn gives the 
ratio of brightness of gas flame divided by brightness of sun for 
the particular wave-length; the fourth column gives the relative 
energy of the same wave-lengths as determined by Langley; and 
the fifth, the product of the third and fourth, shows the relative 
energy distribution in the spectrum of a gas flame. 

Such a calculation would, at best, yield results only approx- 
imately correct, and in this case it is still further open to criticism 
in that Ebert did not use exactly the same wave-lengths as Meyer. 
As was said previously, however, he lays little stress on the shape 
of the visibility curve, insisting only that the maximum lies in 
the green. 

The final threshold values were obtained in relative terms by 
multiplying the values given in column 5 of the above table by 
D?. Since sensitivity is taken as the reciprocal of the threshold 
value of the stimulus, the relative sensitivities of his observers 
were as the reciprocals of these products. The values of these 
reciprocals are given in Table II. 


TABLE II 
Observer Red Yellow Green Blue-Green Blue 
S. 1/25 1/15 1 1/1.3 1/3 
oN 1/34 1/17 1 1/2 1/4 


In 1889 Langley (6) himself published results bearing on the 
visibility of radiation. He wished “to make the novel calcula- 
tion as to the actual amount of energy either in horse-power or 
any other unit, required to make us see.” 


10 MARGARET M. MONROE 


Unlike Ebert, Langley determined the minimum visible by 
means of reflected light. A piece of white paper on a black 
screen was illuminated by spectral light of different wave-lengths. 
As source he used the sun—reflecting its rays into the spectroscope 
by means of a siderostat mirror. The gross reduction of 
intensity was accomplished by reducing the effective area of the 
collimating lens by placing before it a metal plate pierced by a 
minute aperture, by a sectored disc, and by a glass screen very 
lightly smoked. The finer reductions were made by varying the 
width of the collimator slit. By actinometric meastirements, 
Langley found the solar radiation to be 1.5 calories per square 
centimeter per minute. Knowing the reductions made, and esti- 
mating the absorption through the optical system, he calculated 
the values of the minimum visible for four wave-lengths to be as 
shown in Table III. 


TABLE III 
Color Wave-length Energy (ergs) Ratio of Energies 
Crimson 750 pe 1/780 450000 
Scarlet 650 um 1/1600000 230 
Green 550 um 1/360000000 1 


Violet 400 up 1/1500000 240 


These results are for one observer only. The fourth column 
shows the relative energy values of the threshold for the four 
wave-lengths in question. Langley says of these results that they 
are subject to variations of a wide range, and may perhaps be in 
error by as much as 100 per cent. 

The next results bearing on the subject are those of Konig (7), 
who, as part of an extended experiment on the relative brightness 
of different bands in the spectrum at various, intensities, deter- 
mined a threshold visibility curve. He used the Helmholtz spec- 
trum color mixer, reducing the intensity of the light by means 
of slit width. Like Ebert, he used Langley’s figures representing 
the relative energy distribution in the sun’s spectrum, multiplying 
them by ratios previously determined by himself and Dieterici in 
a comparison of the energy distribution of the spectrum of a 
triplex gas burner (the source used) with that of the sun. His 
data are for two observers and fourteen wave-lengths. Table IV 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS Il 


shows the relative sensitivity of the two observers to the wave- 
lengths employed. The method of calculation is similar to that 
of Ebert, described above. 


TABLE IV 


Wave- Observer Wave- Observer 

length A.K. R.R. Av. length A-Kiai kerk. Av. 
670 pee .00019 .00017 .00018 535 up Aa .60 .68 
650 wy .00047 .00056 .00051 520 wm .98 .83 .90 


625 um .0038 .0048 .0043 505 wu 1.00 1.00 1.00 
605 pu .015 .019 .017 490 up .86 5 .80 
590 wu .045 .033 .039 470 wp .50 ait) .50 
575 wm 12 uy “12 450 pp ves .26 25 
555 we .36 ooo ‘35 430 wm .047 .059 .053 


Pfluger (8), in taking up the problem in 1902, adopted a pro- 
cedure similar in principle to that of Ebert. Unlike Ebert, how- 
ever, he determined the energy distribution of the source directly 
with a thermopile, although only in relative terms. Also a 
greater number of observers was used. 

A Nernst filament was used as source, the light from which 
was focussed on the slit of the spectrometer by a condensing sys- 
tem composed of two triple achomatic lenses. This seemed to 
satisfy best the need for a source whose intensity was both con- 
stant and sufficiently great to permit of direct measurement in 
the violet. The energy measurements were made by means of a 
Rubens thermopile mounted at the objective of the spectro- 
scope, and a DuBois-Rubens galvanometer. The radiometric 
apparatus was not, however, calibrated against a standard, and 
the energy curve is, therefore, in terms of galvanometer readings, 
not absolute units. 

In order to allow for the reduction of the light to the threshold 
of sensation, slight changes in the arrangement of the apparatus 
had to be made. The ocular slit was shortened to 34 mm. in 
height, which with the breadth of 14 mm. was considerably 
smaller than the observing pupil. Milk glass was placed over 
the collimating slit to give evenness of field brightness, and the 
focussing lenses removed. The energy measurements were cor- 
rected for the absorption of the milk glass and an attempt was 
made to compensate for the absorption of the lenses by inserting 
glass plates in the beam of light. A bar four meters long was 


12 MARGARET M. MONROE 


added to the collimator arm. The illumination of the milk glass 
could then be varied by changing the position of the Nernst fila- 
ment on this bar. <A further reduction was made by introducing 
a sectored disc into the path of the ray, and the final reduction 
was obtained by varying the slit width. The observer, looking 
through the objective slit, saw the face of the prism illuminated 
with spectral light. To aid in maintaining fixation a milled ring 
was placed at the ocular slit. The diameter of the lighted surface 
subtended an angle of about 12 degrees, its image being, therefore, 
much larger than the fovea, larger even than the macula. 

The procedure was very tedious, says Pfluger. Three read- 
ings were taken for each determination; then the whole series 
was repeated in reverse order. With few exceptions the second 
series was found to correspond to the first within the experimental 
error of the first. When this was not the case the whole series 
was repeated. Errors for single measurements were high, some- 
times reaching 10 per cent. The average error was 4 per cent. 
Results are given for nine observers, and the curves for different 
days are plotted separately. The original intention was to average 
all the results for a given observer, but so much diurnal variation 
was found to exist that the plan had to be abandoned. The ques- 
tion of this large variation from day to day being found simultane- 
ously with a comparatively small variation for any one sitting 
will be discussed later. In a few cases nineteen points in the 
spectrum were employed as stimuli, but the bulk of the curves are 
plotted for eight points. To get the final values the slit width 
readings were averaged, then reduced to what they would have 
been with the source at a distance of one meter and with 360° 
open sector. The reciprocal of this value is taken as the measure 
of the sensitivity. The greatest sensitivity in any series is taken 
as unity, and the other values are made proportional. The posi- 
tion of this maximum sensitivity varies widely, not only from 
individual to individual, but from day to day in the results of. the 
same individual. Three curves selected as typical are reproduced 
in Figures I, II, and III. Figure I gives the curves for an 
observer who shows comparatively little diurnal variation. For 
six days the maximum sensitivity was at 495 wu... There is con- 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 13 


siderable variation, however, at other wave-lengths. Figure II 
gives curves for an observer whose diurnal variation is extreme. 
For example, on two days the maximum sensitivity occurred 
respectively at 525 pp» and 485 wp. Figure III gives the curves of 
an observer in which the tendency toward a secondary maximum 
is pronounced. Varying degrees of this tendency are shown in 
many other curves in Pfluger’s series. From such data it is evi- 
dent that Pfltiger could draw no conclusions as to the exact shape 
of the threshold visibility curve—he could merely give the general 
characteristics of such a curve. His conclusion is: 


“Die absolute und die relative Farbenempfindlichkeit des Auges, gemessen 
bei den Schwellenwerten der Reizempfindung, ist grossen individuellen 
Verschiedenheiten, und, bei demselben Auge, grossem Wechsel unterworfen. 
Die Empfindlichkeit ist am grdéssten fiir den Spectralbereich 1 == 495 uy 
bis A==525 up. Sie kann fiir A717 py, den 33000ten, fiir A413 we den 
60ten Teil des Wertes im Griin betragen.” 


14 





MARGARET M. MONROE 


Figures I-III. Sensitivity Curves (Pfliiger). 


Showing the diurnal variation in achromatic sensitivity of three observers. 
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III 
APPARATUS AND PROCEDURE 


In any quantitative determination of the amount of light 
needed to arouse a just noticeable sensation for any given group 
of wave-lengths there are two essential requirements: (1) We 
must have a means of presenting to the eye the desired range of 
wave-lengths free as nearly as possible from alien wave-lengths ; 
and (2) we must have some means of measuring the energy of 
the stimulus thus presented and of reducing its intensity by 
known amounts. The greater part of the spectroscopic and radio- 
metric apparatus needed to fulfil these two requirements was 
already in use in the Bryn Mawr laboratory when the present 
work was undertaken.” The description of the apparatus as 
modified to meet the needs of the present investigation, together 
with the necessary additions, is given under five headings: the 
source of light; the spectroscope; the apparatus for presenting 
the light to the eye ; the devices for reducing the intensity of light ; 
and the radiometric apparatus. The procedure is described 
under two headings: the energy measurements; and the methods 
of observation. A drawing showing the path of the beam of 
light and the arrangement of the apparatus is given in Figure IV ; 
a photograph of the assembled apparatus in Figure V. 

A. The Source of Light. The source of light was a Nernst 
filament operated at 0.6 ampere. This source was chosen because 
when properly seasoned it gives a light very constant in both 
intensity and radiometric composition, and at the same time 
sufficiently intense to permit of direct energy measurement. Its 
shape also well adapts it for use with the slit of the spectroscope, 
1.e., the shape is such as to make it possible to utilize for the 
illumination of the face of the prism a relatively large part of the 
light emitted. When in use the filament is placed directly in 
front of the slit and as close to it as is possible. This placement 


2For description of this apparatus see References (9) and (10). 
15 


16 MARGARET M. MONROE 


of the filament, however, presents two difficulties. In the first 
place the Nernst material must be heated before it will conduct 
the electric current. This requires that the filament be moved 
from its position in front of the slit prior to each period of work. 
In the second place the terminal wires, which are of platinum and 
very pliable, give little stability of position to the filament. 
Because of these difficulties a special mounting had to be devised 
which would provide for the adjustments required for the removal 
and precise resetting of the filament and would give the rigidity 
of support needed to prevent sagging or other displacement of the 
filament from its position in front of the slit. On this latter 
point it may be noted that if care is not taken that the light which 
enters the slit come always from the same part of the filament, 
variations both in its composition and intensity may occur. This 
mounting is shown in Figure IV. 

B and C are two metal arms at the ends of which are attached 
the terminal wires of the filament N. B and C are supported 
by a piece of asbestos A, which is in turn fastened to the rod D 
by a pin E. E serves a double purpose: by its use B and C are 
supported in a manner which not only provides for both heat and 
electric insulation, but which also allows a slight rotary movement 
necessary for perfect alignment of the filament with the slit. 
The height of D is adjustable and F is fastened by a collar to a 
round rod attached to the collimator arm, thus permitting move- 
ments of the mounting back and forth, right and left, and up and 
down. Around the whole is a metal housing ventilated at top 
and sides, but in such a manner as to remain light-proof. The 
filament is connected in series with a Weston ammeter graduated 
to 0.02 ampere; a ballast which both reduces the current and 
compensates for the change in the resistance of the Nernst with 
change in temperature; and two adjustable rheostats, one coarse, 
the other fine. The former is used to cut down the current to 
approximately the desired value and the latter to correct for the 
fluctuations in the line. 

B. The Spectroscope. A diagramatic representation of the 
spectroscope is shown in the drawing given in Figure IV. Si is 
the collimator slit; Li, the collimator lens; P, the prism; Le, the 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 17 


objective lens; Sz, the objective slit. Ls to Ls is the system for 
focussing the light on the eye. The collimator slit S:1 is 12 mm, 
high, and its width can be varied by means of a micrometer screw 
fitted with a head graduated to read to thousandths of an inch. 
This slit was set at a width sufficient to allow of the radiometric 
measurements being made with precision, and was kept constant 
throughout the experiment. Lenses Li and Le are both Zeiss 
triple achromats, 60 mm. in diameter; the collimator has a focal 
length of 180 mm., the objective of 240 mm. A carbon bisulphide 
prism 105 mm. high, with a refracting angle of 60 degrees, was 
used. With the exercise of a reasonable amount of precaution 
to keep the CSe free from impurities and to maintain a uniform 
temperature in the room, this prism has given satisfaction. If 
the temperature is not kept constant the change of refractive 
index of the CSe, resulting from the change in temperature, neces- 
sitates frequent checking and resetting of wave-length. The 
objective slit is 0.342 mm. wide and adjustable in height. For 
the radiometric measurements a height of 10.4 mm. was used; 
for the work with the eye this was reduced to 1.85 mm. The 
greater length was necessary in order to obtain an intensity suffi- 
ciently high to make the energy measurement; for the eye work, 
however, a much smaller slit is needed in order that the image 
which is focussed on the eye may fall entirely within the pupil. 
This slit is mounted on an independent base screwed to the table 
in a fixed relation to the base of the spectroscope. In order that 
the distance of the slit from the lens Lz may be adjusted for the 
different focal distances of different wave-lengths, the frame on 
which the slit is mounted is furnished with a rack and pinion R. 
Lens Ls, which serves as collimator in the system for focussing 
the light on the eye, is mounted on the same rack and pinion so 
that the distance between Se and Ls (the focal length of Ls) 
remains always constant. 

In order to obtain automatically minimum deviation for all 
wave-lengths falling on the objective slit, the spectroscope was 
fitted with a special attachment for the purpose. K in Figure IV 
is a rod fastened to the prism table in such a position as to be 
continuous with the radius of the table which bisects the refracting 


18 MARGARET M. MONROE 


angle of the prism; X and Y are two rods of equal length 
fastened at one end to the two arms of the spectroscope at points 
equidistant from the center of the prism table, and at the other to 
a collar Z, which is free to play back and forth along rod K. M 
is a micrometer screw with a graduated head which is used to 
move the collimator arm through the small angles needed to 
change the wave-length. Opposite this screw is a plunger work- 
ing against a spring. The collimator arm is held between the 
screw and the plunger so that it responds to a movement of the 
screw in either direction. By this attachment the prism is always 
turned through half the angle traversed by the collimator arm in 
changing the wave-length. Therefore if the prism is once set 
for minimum deviation for the D-line, there will also be minimum 
deviation for any other wave-length. That is, when the prism 
is set for minimum deviation, the line bisecting the refracting 
angle of the prism also bisects the angle made by the incident and 
emergent rays, hence if in changing the wave-length the angle 
between the incident and emergent rays be changed a given amount 
by a movement of the collimator arm, the prism must be moved 
through half that angle in order that the line which bisects its 
refracting angle will also bisect the angle made by the incident 
and emergent rays. 

In all quantitative work on color sensitivity it is very important. 
that the light employed be as homogeneous as possible as to wave- 
length. The presence of alien visible wave-lengths affects the 
determination of chromatic sensitivity in two ways: (1) It 
decreases the amount of the color response through physiological 
inhibitions and interactions, and (2) it increases the value of the 
energy measurements. In the work in this laboratory determina- 
tions made with and without provision for absorbing impurities 
SOUIIIYIP MOYS ‘939 ‘SUOTPOHoI [eUIOJUT ‘GYSI] poi9}jeVos 0} onp 
large enough to be considered significant. In the present investi- 
gation the aim has been to obtain a degree of purity such that any 
portion of the spectrum used should show only one band when 
examined with a second spectroscope. In order to secure purity 
of light the following precautions were taken. The spectroscope 
was provided with the minimum deviation attachment already 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 19 


noted. Great care was employed in eliminating as far as possible 
all stray light and internal reflections. The source was housed 
as described above and a screen was placed at the objective lens to 
prevent any stray light from the prism from reaching the farther 
parts of the system. As far as could be all surfaces that might 
either admit or reflect extraneous light into the path of the 
refracted beam were blackened, and as a final precaution a light- 
tight housing was built around the whole apparatus from just in 
front of the collimator slit to just beyond the lens which focussed 
the light on the eye. This compartment was large enough to 
permit of the experimenter working inside. Even with these 
precautions, however, some impurities remained, chiefly those due 
to reflections from the surfaces of the lenses. These were 
absorbed out by very thin gelatine filters, carefully selected with 
reference to the bands to be eliminated. The gelatine filters were 
held in place over the objective slit by small clips fastened on 
either side of the plate containing the slit. The filters were used 
both for the radiometric measurements and for the threshold 
determinations. 

C. Apparatus for Presenting the Light to the Eye. ‘There are 
several methods, by which we may obtain a homogeneously illumi- 
nated surface suitable for determining thresholds of sensation: 
(1) Spectrum light of the desired range of wave-lengths may be 
allowed to fall on some diffusively reflecting surface, such as 
magnesium oxide, which in turn is viewed by the eye; (2) spec- 
trum light may be allowed to fall on some diffusely transmitting 
surface; or (3) spectrum light may be focussed directly on the 
pupil of the eye by means of a double convex lens. The third of 
these possibilities was chosen for this work. The use of either 
of the first two methods necessitates the assumption that the 
reflecting or transmitting surface is absolutely nonselective to 
wave-length. Both methods, moreover, are very wasteful of 
light—not only is there a comparatively high percentage of 
absorption, but also only a small percentage of the light coming 
from any point of the stimulus surface enters the pupil of the eye. 
Under these conditions it would be very difficult to specify accu- 
rately in radiometric units either the unit density or the total 


MARGARET M. MONROE 








KH Wt 9. § | $4" 40g 


Pare etree EPEC VED EERE EOCENE 


\ 





THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 21 


amount of the light at the eye. That is, the energy value at the 
eye could not be measured; it could be calculated only approx- 
imately. An accurate energy specification is possible, however, 
when the light is focussed on the eye. The light, by this method, 
does not spread from the stimulus opening as if emanating from 
a source, but is concentrated into an image on the pupil of the 





FiGuRE V 


eye—an image of the objective slit of the spectroscope. The 
amount of energy concentrated into this image can readily be 
determined with the radiometric apparatus to be described later. 
In order to present the stimulus in compliance with the above 
plan, the rays of light emerging from the objective slit are first 
rendered parallel by lens Ls (Fig. IV) placed at its focal length 
(150 mm.) from the objective slit, and are then focussed on the 


22 MARGARET M. MONROE 


eye by lens La, focal length 275 mm. In this way ample space 
is obtained for the introduction into the system of any apparatus 
necessary for reducing the intensity of the beam of light, 1.e., 
sectored disc, filters, etc. The dimensions of the image at the 
eye were determined by photographing it on a plate carefully 
mounted in the plane of the pupil and measuring the photo- 
graphed image with a micrometer comparator. These dimen- 
sions were 3.7326 mm. X 0.6956 mm. __ Since this is well within 
the pupil of a dark-adapted eye, the use of an artificial pupil with 
its attendant difficulties is avoided. 

In front of Li, 240 mm. from the eye, a screen G is placed 
containing a circular opening O, 10 mm. in diameter, which serves 
to diaphragm the lens Ls to the desired stimulus aperture. When 
the eye is in position the exposed area of the lens Ls is seen filled 
with light. The visual angle subtended by this area is 2° 2.2’. 

D. Means of Reducing the Intensity of Light. Because of the 
small amount of energy required to arouse a just noticeable sen- 
sation of light, it would be impossible to measure this energy | 
directly with a thermopile. It is necessary, therefore, to measure 
the energy at a high intensity and to reduce this intensity by 
known amounts to the intensity required. In order to obtain the 
range of reduction necessary for all observers, three methods of 
cut-down were utilized. 

1. The Filters. The main reduction of light was made by 
the insertion in the light beam of neutral filters, combinations of 
which allowed for a possible range of transmission of from 
1148 « 10° to 141 « 10°. The filters are of gray gelatine 
mounted between glass, 25 mm. by 25 mm. They were made by 
the Eastman Kodak Company. The densities were specified by 
the Eastman Kodak Company and the transmission of each filter 
was calculated by the formula: 

1 
Density = log 
Transmission 


A holder designed to take any combination of filters up to 
eight was placed at H in contact with the screen G. This position 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 23 


of the filters precluded the possibility of any scattered light not 
completely absorbed being transmitted through the screen opening 
to the eye of the observer. 

2. The Sectored Discs. Any further large reduction needed 
was obtained by the use of a pair of sectored discs inserted at SD. 
These discs were rotated by a small motor suspended from above 
by coiled springs to absorb vibration. The range of open sector 
used was from 180° to 0°. 

3. The Wedges. The final reduction was made by means of 
the two wedges WW mounted immediately in front of the objec- 
tive slit. Early in the work it was realized that for any accurate 
threshold determinations a means must be had of making very 
small changes in the intensity of the stimulus, and that the method 
employed must insure a perfectly uniform reduction throughout 
the cross section of the beam of light. A single wedge such as is 
ordinarily employed does not give this required uniformity of 
reduction. When placed in front of the slit, no matter how fine 
the gradation in density, there is always a slight difference in 
transmission between the opposite edges of the used portion of a 
single wedge. A double wedge device was therefore planned to 
obviate this difficulty. The two wedges were made according to 
specification and calibrated by the Eastman Kodak Company. 
Like the filters, they are of neutral gelatine mounted between 
glass. The wedges are identical, each being 135 mm. by 13.5 
mm., and so constructed as to cover the same range of transmis- 
sion. They are mounted parallel to each other in holders which 
are operated by a micrometer screw. These holders are provided 
with right- and left-handed threads. As the screw is turned the 
wedges move in opposite directions, each wedge traveling in front 
of the other through a path equal, if need be, to twice the length 
of one wedge, that is, from a position of juxtaposition at the thin 
end of each wedge to a position of juxtaposition at the thick end. 
Since the density gradients of the two wedges are identical and 
the wedges move in opposite directions, it is obvious that the 
resultant densities will be the same from point to point throughout 
the overlapping section. A consideration of the range of move- 
ment shows further that a series of densities may be obtained 


24 MARGARET M. MONROE 


varying by minute amounts from the sum of the minimum densi- 
ties of both wedges, through the sum of the minimum of one and 
the maximum of the other to the sum of the maxima of both. 


1 


The density in this case also equals the log of 
Transmission 


E. The Radiometric Apparatus. The apparatus used con- 
sisted of a linear thermopile of silver and bismuth couples, a 
Paschen small coil galvanometer especially constructed for the 
thermopile employed, and suitable auxiliary apparatus. These 
instruments were constructed by W. W. Coblentz of the Radio- 
metric Division of the Bureau of Standards. The apparatus has 
been described in an article by Dr. Ferree and Dr. Rand (10), and 
the reader is referred to this article for further details. All the 
radiometric measurements were made by Dr. Rand, to whom I 
am deeply indebted for the values given below. The procedure 
of making these measurements is quoted from the article just. 
mentioned. 

F. The Energy Measurements. The apparatus for measuring 
the energy is so planned that measurements may be made at the 
objective slit, at the stimulus opening, and at the eye. A descrip- 
tion at one of these places, namely, the objective slit, is sufficient 
to show in a general way the method employed. 

“The thermopile to be used was placed in position immediately 
behind the slit and a blackened aluminum shutter was interposed 
in the path of the beam of light between the slit and the end of the 
objective tube of the spectroscope. Preliminary to the exposure 
of the thermopile to the light to be measured, -the current sensi- 
tivity of the galvanometer was tested by means of a special device 
provided for this purpose in the construction of the galvanometer. 
With regard to this procedure it may be pointed out that the 
current sensitivity of the galvanometer varies with the period or 
time of the single swing of its needle system. Since it is not 
possible to control the field so as to get this period always the 
same, it is necessary, if results are to be compared, to take some 
sensitivity as standard and to convert all readings into deflections 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 25 


for the standard sensitivity by means of a correction factor deter- 
mined at each sitting. (For a detailed description of the method 
of determining this factor, see PsycHoL. Rev. Monoa., 1917, 24, 
No. 2, pp. 60-65. ) 

“The thermopile was next connected with the galvanometer 
and the light allowed to fall on its receiving surface until a 
temperature equilibrium was reached (ca. 3 sec. for our thermo- 
pile). The deflections were read by means of the telescope and 
scale and the readings are corrected to standard sensitivity by 
means of the factor previously determined. The final step in 
the process of measuring was the calibration of the apparatus, 1.e., 
the value of 1 mm. of deflection in radiometric units was deter- 
mined for the area of thermopile exposed. To do this a radiation 
standard, the value of the radiations from which is already 
known, had to be employed. The standard used by us was a 
carbon lamp specially seasoned and prepared for the purpose by 
W. W. Coblentz. This lamp was placed on a photometer bar 
2 meters from the thermopile and operated at one of the intensi- 
ties for which the calibration was made, in our case 0.40 ampere. 
The thermopile was exposed to its radiations with the same area 
of receiving surface as was used in case of the lights measured, 
and the galvanometer deflection was recorded. From the deflec- 
tions obtained the value of 1 mm. of deflection, or the radiation 
sensitivity of the apparatus under the conditions given, was com- 
puted from the known amount falling on the surface of the 
thermopile. Having the factor expressing the radiation sensi- 
tivity of the apparatus, the deflections produced by the wave- 
lengths of light measured were readily converted into energy 
units.” 

The radiation sensitivity of the linear thermopile as used in 
the present investigation was computed from the following data. 
The energy value of the radiations per square millimeter of receiv- 
ing surface from the standard lamp at a distance of 2 m. operated 
at 0.40 ampere was 90.70 & 10°° watt. The deflections of the 
galvanometer produced by this intensity of radiation falling on 
the same area of receiving surface as was used in measuring the 
lights employed as stimuli, when corrected (a) to a sensitivity 


26 MARGARET M. MONROE 


of i 1 10°'° ampere, and (b) for the absorption of the glass 
cover of the thermopile, was 323.85 mm. The area of the sur- 
face exposed was 3.5657 sq. mm. The value of 1 mm. of gal- 
vanometer deflection, or the sensitivity of the instrument for the 
area of receiving surface used, was, therefore, 998 & 10-™ watt, 
By means of this factor the galvanometer readings produced by 
the different wave-lengths of light were readily converted into the . 
energy value of light falling on the receiving surface of the 
thermopile. For the purpose of the present investigation, how- 
ever, it is needed to know also the energy values of the light 
entering the eye. These are sufficiently great only in the case 
of red and orange to be measured directly with the required pre- 
cision. It was necessary, therefore, to measure all the wave- 
lengths used at the objective slit and only the red or orange at 
the eye, and from the comparative values of the red or orange at 
the two places to determine a correction factor which will repre- 
sent for all the colors the reduction of the light from objective 
slit to eye. In order to determine this reduction factor with 
precision a larger area of receiving surface of the thermopile had 
to be exposed to the light than the actual area of the image enter- 
ing the eye for the threshold determinations. It will be remem- — 
bered that the height of the objective slit and consequently the 
height of the image focussed on the pupil of the eye was adjust- 
able. In the present work the receiving surface of the pile used 
to measure the light at the eye was 11.2548 & .895 mm., or 10.073 
sq.mm. The value of 1 mm. deflection of the galvanometer for 
this area of receiving surface was 88 < 10°'° watt. Since the 
focussed image is of uniform density, the energy for the area of 
the image at the eye, 3.7326 mm. & .6956 mm., or 2.5967 sq. mm., 
could be readily determined. From these values a single factor 
was calculated which would convert the amount of energy for 
the different wave-lengths measured at the objective slit into 
the amounts of energy entering the eye. These values are given 
in Table V. A division of these values by 2.5967 will give the 
energy density at the eye, a division by 78.54 the energy density 
at the stimulus opening. 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 27 


TABLE V 
Red 655 uu = 300 xX 10-9 
Orange 616 wp = 133 yy 
Yellow 580 npu= 16.2 Zn 
Yellow-green Shey Pees 4g 
Green 522 uu= 14.8 nf 
Blue-green 489 pu= 9.86 “ 
Blue A653 op O19 


G. The Methods of Observation. The experiments were con- 
ducted in a light-tight dark-room, the walls, floor, and ceiling of 
which were painted black. In the preliminary work it was found 
that an adaptation period of twenty minutes was sufficient to give 
constant results in the determination of chromatic thresholds. 
For the achromatic thresholds a much longer adaptation period 
was needed. The observer was seated in front of the apparatus, 
the eye being at the focal length of the lens Ls. To ensure steadi- 
ness of position the head was held rigid by means of a wax- 
coated mouthpiece in which the impression of the teeth had previ- 
ously been made and hardened. The other eye was lightly ban- 
daged with a black cloth. The determinations were made in 
ascending and descending series. The edges of the stimulus 
opening were touched at suitably spaced intervals with luminous 
paint to enable the observer to take and hold the correct fixation. 
A rough adjustment of the filters and sectored disc was made until 
an approximate value of the threshold was obtained, and then a 
rest period of several minutes was given and the exact value of 
the threshold accurately determined. In order to prevent a pro- 
gressive loss of sensitivity from fatigue short rest periods were 
given after each observation. A number of independent deter- 
minations were made of the threshold value for each wave-length. 

A similar procedure was employed for the determination of the 
achromatic threshold for each wave-length. In this case, how- 
ever, even greater care had to be exercised to guard against 
progressive loss of sensitivity. 


IV 
STATEMENT AND DISCUSSION OF RESULTS 


A. Achromatic Thresholds. The achromatic thresholds of the 
seven wave-lengths were determined for twenty-one observers. 
A complete statement of the results is given in Table VI, Parts 
A and B. A graphic analysis of Table VI is given in Figures 
VI-XIV. 

The minimum visible for the different wave-lengths used was 
found to be as follows: 


1. Red (655 up) Average = 626.99 watt X 10-16 
Median 6. + 


37 .67 
1209.27 — 186.48 ¥ 


Range = 
2. Orange (616 up) Average= 130.28 < 
Median = 118.08 4; 
Range == 273.77 — 70.72 $ 
3. Yellow (580 up) Average 27.96 € 
Median = 25.74 pe 
Range = 49.04 — 11.56 . 
4. Yellow-Green (553un) Average 2.203 . 
edian == 2.298 © 
Range) )==) 3242 —'), af718 7 
5. Green (522 up) Average= 1.953 
Median = _ 1.868 oe 
Range = 3.469— 1.295 , 
6. Blue-Green (489 up) Average=— 8.11 * 
Median = 8..00 Hs 
Range = 13.8 — 2.96 7 
7. Blue (463 pu) Average= 15.62 + 
Median = 15.00 > 
Range = 29.53 — 6.28 +: 


The total range of minimum visible of the wave-lengths used 
is therefore from 1209.27, in the red, to .718 (watt X 107%*) in 
the yellow-green, a ratio from highest to lowest of 1684. 

The average sensitivity of the twenty-one observers was great- 


est in the green. 


1.953 (watt X 107°). 


The average threshold for this wave-length is 


There is not, however, complete uni- 
formity as to the position of maximum sensitivity. 


Nine observ- 


Part A of 


ers show a maximum sensitivity in the yellow-green. 


Table VI gives the results of those whose maximum sensitivity 
28 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 29 


TABLE VI 


ACHROMATIC THRESHOLDS 
Amount of light entering the eye (watt X 10-16) 


Yellow- Blue- 
No. Observer Red Orange Yellow Green Green Green’ Blue 
1. M.M.M. 922.41 103.53 45.74 2.208 1.405 9.58 15.23 
2a 787.50 192.70 11.56 3.145 1.981 10.97 25.95 
3. Md.B. 753.63 70.72 44.61 2.596 1.471 11.25 16.37 
4. M.B. 689.28 91.14 32.13 2.107 1.295 3.83 8.16 
S.C. 667.50 129.52 35.06 3.242 2.446 7.16 19.55 
Gavel: 648.83 A I ds 59.50 3.220 1.868 8.00 11.06 
» ola Bal ASE 637.67 273.77 25.74 2.665 lee 7.74 16.00 
8 3.G.L. 626.22 106.24 31.08 2.586 2.116 8.46 14.14 
9. M.O.L. 613.90 136.22 21.94 3.024 1.939 7.65 23.302 
LUN Ce Gea 6 609.84 123.17 49.04 2.512 2.268 10.50 18.31 
135 T WL. 498.96 141.90 23.08 1.661 1.513 6.04 20.56 
12. Md.B.L. 461.16 115.64 28.73 2.298 2.059 9.91 8.90 
Mean— 659.74 141.76 32.02 2.605 1.840 8.42 16.46 
Median—A 643.25 126.35 31.61 2.591 1.904 8.23 16.18 
; B 
13.:J.G: 1209.27 118.08 25.56 2.480 3.450 8.26 15.06 
14. T.W. 787.20 176.08 21.83 1.293 1.353 eT 13.18 
15. M.MC. 642.33 108.10 32.06 2.264 2.539 5.07 11.04 
163 Ei: 641.28 88.20 20.74 818 1.548 6.36 12.20 
17. M.G.R 597.00 119.36 21.14 2.203 3.060 13.80 23.19 
18. M.O. 593.45 98.42 29.52 2.550 3.469 10.66 29.53 
19. R.N. 305.71 135.42 14.38 1.622 1.718 7.79 13.00 
AY. WO Le 287.30 79.72 18.18 718 1.395 2.95 7.07 
Zipetivks 186.48 111.55 19.66 1.066 1.416 3.10 6.28 
Mean—B 583.34 114.99 22.56 1.668 2.216 7.68 14.51 
Median—B 597.00 PEE5S 21.14 1.622 1.713 7.79 13.00 


Mean—A and B 626.99 130.28 27.96 2203 1953 811 15.62 
Med.—A and B_ 637.67 118.08 25.74 2.298 1.868 8.00 15.00 
lies in the green, Part B the results of those whose maximum 
sensitivity lies in the yellow-green. With two such distinct types 
a single measure of the general tendency of the group is mislead- 
ing—the mean and median of the two groups were therefore 
calculated separately. As shown by these measures, the two 
types are quite different not only with regard to the position of 
maximum sensitivity but also with regard to the absolute value of 
the thresholds. With the exception of the green, the average 
threshold values of the yellow-green type are smaller than those of 
the green type. There is, however, on the whole, a wider range of 
values in the yellow-green type than in the green type. 

In order to show more clearly the difference between these two 
groups, as well as to show the relative average sensitivity through- 
out the spectrum, the results of Table VI, A and B, were plotted 


30 MARGARET M. MONROE 


in the form of sensitivity curves. The reciprocal of the energy 
value was taken as a measure of the sensitivity. The maximum 
sensitivity for each observer was then made equal to unity and the 
other six values for the observer calculated as ratios of that value. 
Figure VI is the composite of the twenty-one sensitivity curves. 
In this composite we can distinguish not only the two groups 






Achromatie Sensilivily 








Sada ee 


a oy 
Filla SS 
Wate 






Wave Length 





655 b 16 580 553. 522 9463 
Ficure VI . 


under discussion, but a possible third group. Two observers, 
although belonging to the type having maximum sensitivity in 
the yellow-green, are also very sensitive to green, as is indicated 
by the great breadth of the curve between wave-lengths 522 pu 
and 553. Similarly a few observers belonging to the type 
having maximum sensitivity in the green are very sensitive in the 
yellow-green. It is possible that the true maximum in both these 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 31 


cases may be neither green nor yellow-green, but some intermedi- 
ate wave-length. If the investigation in hand had been for the 
purpose of determining the exact shape of the threshold visibility 
curve for all types of observers, the region between 522 pp and 
553 u» would have been carefully explored for all cases differing 
from the average in this way. The purpose of the study was, as 


has been stated, the determination of the minimum visible at 
1.0 





8 : | 
a | 
6 : 
= al 
= { 
5} 
« rs H 
c : 
a” ‘ f 
ep Meet .f 
= : f — Average 
Tw 6 h : 
3/5 \ |i ----Median 
be : 
<— \ 
#\< ‘ 
J Z 
ee” Wave Length 
2 655 er zig 580 §&53 522 489 463 pH 
Ficure VII 


certain representative points throughout the spectrum. The pos- 
sibility of a third type is offered, therefore, merely as a suggestion 


as yet unproved. 


The mean and median values of the two groups were also 
calculated in the form of ratios of sensitivity. 


Figure VII gives 
these average sensitivity curves. 


The curves for the median and 
mean values agree very closely for the green type, and also for 


the yellow-green type except in wave-length 522 py. 


32 MARGARET M. MONROE 


For all the wave-lengths employed the distribution of values 
around the average is approximately symmetrical. The fre- 
quency graphs for the seven parts of the spectrum used are given 
in Figures VIII-XIV. The size of the class interval for the 
vertical codrdinate, representing the number of observers, is the 
same in all the curves. That along the horizontal coordinate 
differs according to the absolute value of the minimum visible 
in the particular color. Thus in red, where the value of the 
minimum visible is large, the interval is 400 (watt x 10°), 


16 
4 
12 
“a 
os 
f @& 
UNS Bites 
wo 
n 
=) 
§}° 
— 
o 
= 
6) 5 
= 
> 
on 4 
2 





-/6 ) 


Energy -(Walt x 10 | 
1250 850 450 50 
Ficure VIII. Distribution of threshold values for red, 655 wn (Table VI) 





while in green it is only .8 (watt X 10°°*). The intervals were 
further chosen so that the median threshold value of the group in 
question would fall approximately at the center of some interval. 
Since the number of observers was small, it was thought that a 
truer picture of the distribution of values was obtained by the 
use of rather large intervals. As has been said, the frequency 
graphs are very nearly symmetrical—each shows a large average 
group and two smaller, almost equal groups, .one superior, one 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 33 


inferior to the average. Since this is true even for a small 
group, it seems reasonable to suppose that the same would be 





Number of observers 






Energy - (watt x107!®) 
216 184 q ° 
Ficure IX. Distribution of threshold values for orange, 1616 uu (Table V1) 





Number of observers 





Energy-C Watt 107") 


50 34 13 2 
Ficure X. Distribution of threshold values for yellow, 580 yu (Table VI) 





34 MARGARET M. MONROE 


true for normal observers as a whole. In other words, it is 
probable that there is no uniform “normal”’ sensitivity to light 
of different wave-lengths, but a very wide range of sensitivity 
within which an individual threshold may be considered normal. 
B. Chromatic Thresholds. It was originally hoped to deter- 
mine the chromatic and achromatic thresholds of the seven wave- 






Number of observers 


™N 





Energy- (Watt x107!®) 


4.6 3.2 1.8 A 
Ficure XI. Distribution of threshold values for yellow-green, 4553 us 
(Table VI) 


Number ot observers 


Energy-(Watt x 107!) 


3.5 2.5 1.5 5 
Ficure XII. Distribution of threshold values for green, A522 uu (Table VI) 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 35 


lengths for the same observers. Most observers, however, were 
unable to give the time required for this, and comparison is pos- 
sible, therefore, only between averages. The values of the 
chromatic thresholds obtained are given in Table VII. Since in 
only a few cases did the same observer determine the chromatic 
threshold for all seven wave-lengths, each column of Table VII 
gives the twenty-one values obtained arranged not according to 





nn 
— 
qa 
> 
x. 
fob) 
wo 
oe mej 
o 
i 
°o 
a 
1 & 
© 
= 
=> 
= 
; Energy- (Watt x 1o7!*) 
14 10 6 2 
Ficure XIII. Distribution of threshold values for blue-green, \ 489 pu 
(Table VI) 


Number of observers 





16) 


Energy- (Walt x 10° 


30 20 10 0 
Figure XIV. Distribution of threshold values for blue, 4463 uu (Table VI) 


36 MARGARET M. MONROE 


observers, but according to order of magnitude. The results are 
presented graphically in Figures XV—XXIII. 

The minimum visible—chromatic—for the different wave- 
lengths used was found to be as follows: 


1. Red (655 up) Average __.1778 watt X 10-12 
Median = ..159 ~ 
Range = .460 — .0597 “ 
2. Orange (616 up) Average  .562 a? 
Median = __.166 A 
Range = 3.11 — .054 3 
3. Yellow (580 up) Average 9.57 
Median = 4.32 
Range =52.8 —.190 . 
4. Yellow-Green (553 un) Average .0856 . 
Median — _ .0127 a 
Range = _ .621 — .00771 fe 
5. Green (522 uu) Average .143 5 
Median = .0436 re 
Range = _ .603 — .0042 - 
6. Blue-Green (489 un) Average _ .643 3 
Median = __ 460 - 
Range = 3.16 — .0299 5 
7. Blue (463 uu) Average __—.812 x 
Median = _ .329 e 
Range = 3.07 — .0298 og 
TABLE VII 


(CHROMATIC THRESHOLDS 
Amount of light enternig the eye (watt < 10-12) 
Yellow- Blue- 


No Red Orange Yellow Green Green Green Blue 
1 460 Zire Ol 52.8 .621 .603 3.16 3.07 
2 .370 2.44 49.5 413 .587 2.91 2.54 
3 316 1.84 38.9 9 .504 986 2.48 
4 301 833 1333 0968 401 925 Dae 
5 228 820 8.57 .0959 398 714 bP 
6 188 522 7.31 .0870 0859 48 1.14 
7 171 432 7.07 .0268 0684 504 814 
8 168 242 6.09 0148 0493 482 566 
167 227 4.95 0142 0474 473 433 
10 163 170 4.61 0130 0449 460 355 
11 159 166 4,32 0127 0436 460 329 
12 155 163 998 .0125 0429 366 284 
13 129 149 896 .0119 0420 351 274 
14 127 135 596 .0117 0344 343 257 
15 117 118 546 .0108 0218 321 252 
16 101 0931 519 .0107 00818 233 
17 100 0859 516 .0100 00628 0477 213 
18 0999 0786 516 00959 00497 0473 0574 
19 0832 0667 463 00890 00449 0404 0425 
20 0715 0568 461 00853 00420 0321 0392 
21 0597 0540 .190 00771 00420 0299 0298 
Mean .1778 .562 9.57 .08555 .14314 643 .812 


Med. — .1590 . 166 4.32 01270 .04360° .460 329 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 37 


The total range of the chromatic minimum visible of the wave- 
lengths used is therefore from 52.8 in the yellow to .0042 
(watt < 10°?) in the green, a ratio from highest to lowest of 
12571; 

The average chromatic sensitivity is greatest in the yellow- 
green. The average threshold value for the yellow-green is 


Average — 
Median --- 





FIGURE XV 


08556 (watt x 10°*). The relative sensitivity to the other 
wave-lengths is best shown by Figure XV, which gives the mean 
and median values in the form of sensitivity curves. These 
curves were plotted in the same way as those in Figure VII. 
In both the mean and median sensitivity curves there is an 
extremely sharp drop from the yellow-green to the yellow. Since 
the chromatic thresholds for all colors were not determined 
throughout on the same observers, a composite of the individual 
sensitivity curves, similar to the composite of achromatic sensi- 


38 MARGARET M. MONROE 


tivity curves could not be made. All seven thresholds, however, 
were determined for six observers, and a composite of their 
sensitivity curves is given in Fegure XVI. There is much more 
variation in these curves than in those representing achromatic 
sensitivity. Four only have a maximum sensitivity in the yellow- 
green; of the other two, one shows a maximum in the green, the 
other in the blue. The relative sensitivity to the longer wave- 


1.0 


mS 
Chromatic Sensilivily 





lengths is more uniform—all six curves show the drop in the 
yellow, and all are fairly close together in the orange and red. * 
For the observers tested, the distribution of chromatic threshold 
values around an average is not symmetrical. In each color the 
threshold value of the median is much smaller than that of the 
corresponding average—that is to say, the curves are skewed to 


a 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 39 


the upper end. This means that the range of values below the 
median is much greater than that above the median. For 
instance, there is a difference of 48.48 & 10°” watt between the 


Number of observers 





Wx 10 ~!2 W x 1907'3 
Figure XVII. Showing the threshold values for red, 4655 uu (Table VII) 


an” 
Kas 
aw 
> 
5 
ae 
wn 
2 
(ae) 
S 
Lee 
aw 
pt ew ] 
e 
—_ 
z 





WX 10TH WX 10TH W107 


Figure XVIII. Showing the threshold values for orange, \ 616 uu (Table VII) 


40 MARGARET M. MONROE 


median and the largest threshold in yellow, while there is a differ- 
ence of only 3.13 between the median and the smallest threshold 
value in the same color. Because of this very wide range it was 
thought advisable to plot the logarithms of the energy values of 


Number of observers 





WES 10 5fe (NX EO A WX 107! 
Figure XIX. Showing the threshold values for yellow, 4580 un (Table VII) 


an 
ft 
@ 
> 
. 
au 
1 
a 
i=) 
Co 
oa 
u 
wy 
_ 
f= 
| 
Zz 





WX 107% WW. X10T Wx 107% 


Figure XX. Showing the threshold values for yellow-green, \553uu 
(Table VII) ‘ 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 41 


the chromatic thresholds rather than the values themselves. 
Figures X VII—XXIII shows the frequency graphs of the seven 
wave-lengths. Along the ordinates is plotted number of cases. 
As in the curves for achromatic sensitivity, the class interval 


Number of observers 





WX 1072 WXI073 Wx 107! 
Figure XXJI. Showing the threshold values for green, 4522 uy (Table VII) 


2 
c 
wy 
> 
, = 
@ 
wv” 
2 
° 
cio 
o 
ae 
qw 
2 
e 
> 
2 





W.X 107" WX 107% W.X 1078 


Figure XXII. Showing the threshold values for blue-green, \ 489 wu 
(Table VIT) 


42 MARGARET M. MONROE 


for the abscissae differs for the wave-lengths. Thus the first 
interval of the abscissa in blue-green, blue and orange contains 
all cases of the order 10°" watt, the second interval all cases of 
the order 10° watt, and the third all cases of the order 107% 


watt. In green and yellow-green the intervals contain cases, 


of the orders 10”, 10°, and 10°“ watt, respectively, and i in the 
yellow 10°*°, 10°", and 10°?” watt. 

It will he noted that the graphs of red and yellow differ from 
those of the other five wave-lengths by showing skewness even 


ey 


| 






10 

Me 

= 

wv 

> 

g p 
a 

ce) 

_ 

6 oS 
a 

co) 

i 

ioe) 

+ —< 
& 

> 
2,2 





Energy 


W. X10 7" WX {O72 WX 107!3 
Figure XXIII. Showing the threshold values for blue, \ 463 uu (Table VII) 


in their logarithmic form. In red there is no inferior group— 
the values fall into a large average group and a smaller superior 
group. In yellow there is no superior group, but there is an 
additional very inferior group. It might seem reasonable to 
account for this on experimental grounds because of the differ- 
ence in the abruptness with which the chromatic component comes 
into the sensation in the two cases. The chromatic threshold for 
red is by far the easiest to determine. The transition from 
achromatic to chromatic is abrupt and sharply marked and there 
is little or no hesitation on the part of the observer as to whether 
or not there is any color present. The ease of judgment resulting 
from this small and clearly defined transition interval probably 


ae 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 43 


accounts for the absence of the inferior group. In yellow just 
the opposite condition obtains. The color comes in very slowly 
and gradually and the exact point of appearance of the chromatic 
component is difficult to determine. At low intensities light of 
wave-length 580 wz has much the appearance of ordinary artificial 
light, such as that emitted by a carbon lamp, which an unpracticed 
observer is accustomed to consider as white. In such a case 
much greater intensities are needed for the light to be called 
colored. This may account for the additional very low group in 
yellow not found in any other of the colors. 


C. The Photochromatic Interval. By the photochromatic 
interval is meant the colorless interval between the chromatic and 
the achromatic thresholds. In the present study the actual energy 
of both thresholds has been measured. This makes it possible to 
express the value of the photochromatic interval in absolute 
terms—terms that admit of a numerical comparison from wave- 
length to wave-length. Previous to this the photochromatic 
interval has been discussed only in the most general terms. Its 
existence, and the fact that it varied under different conditions, 
were noted by Tschermak (11), Fick (12), v. Kries (13), Her- 
ing (14), and others. There has been only one numerical state- 
ment of the value of the colorless interval between the two sets 
of thresholds, that of Charpentier in 1888 (15). Charpentier 
focussed light from the spectrum of the sun on ground glass, 
using the colored surface thus obtained as the stimulus for his 
determinations. The intensity was reduced by a diaphragm. No 
measurement, either photometric or radiometric, was made of the 
actual intensities used. The relation between the chromatic and 
achromatic thresholds was taken as equal to the ratio of the square 
of the diameters of the diaphragm openings necessary for the two 
thresholds at any given wave-length. The diameter of the open- 
ing required to reduce light in the yellow to the achromatic 
threshold was 1, the opening for the chromatic threshold of the 
same color was 3.1. The chromatic threshold is then, according 
to Charpentier, 9.6 times as large as the achromatic threshold. 
The ratios found are as follows: 


44 MARGARET M. MONROE 


Rouge "extremes... Sere ek eee eee re 3.6 
Orange sts sey. elles haar ene ib areas fee arene 5.5 
Jarre ys is vs ce ates « Le een Oe ag el tame ina 9.6 
Vert (mMoyennsles t's sae peige hase eingeiteree eee ee 196.0 
Blew franc, région moyenne: sa... ss.01- sence. 635.0 


The wave-length is not stated. Such ratios, obtained without 
reference to the absolute or even the relative intensity of the lights 
used, have little value other than to indicate that there is a 
photochromatic interval. . 

In 1892 Abney and Festing (16) determined the achromatic 
and chromatic thresholds throughout the spectrum and attempted 
to give a photometric evaluation to the results obtained. A mono- 
chromatic beam and a comparison white beam were so reflected 
as to fall on adjoining portions of a white screen. The chromatic 
thresholds were determined by reducing the intensity by means of 
a sectored disc introduced into the path of the monochromatic 
beam. Relative luminosity values were calculated by the use of 
a spectrum luminosity curve previously obtained. Relative lumi- 
nosities, however, change with change in intensity of light, and 
this curve, while a low intensity curve, was not determined for 
threshold intensities. Achromatic thresholds were similarly 
determined, the relative luminosities of the different wave-lengths 
being calculated from the sectored disc values and the luminosity. 
curve just mentioned. Abney and Festing did not, however, 
attempt to assign any values, either relative or absolute, to the 
photochromatic interval as such, and since their values for the 
thresholds are only relative, it is not possible to calculate it from 
their data. 

The values of the photochromatic interval for the seven wave- 
lengths used in this study are shown in Table VIII. Column 2 
gives the energy value of the minimum visible achromatic, column 
3 that of the minimum visible chromatic, and column 4 the differ- 
ence between the two. In column 5 the values of column 3 are 
given in the form of ratios, the value for yellow being made equal 
to unity. The values are very large, ranging from 853.29 & 10°* 
watt in the yellow-green to 95672.04  1077® watt in the yellow. 
There is, apparently, a great difference between the development 
of the color sense of the eye and that of the light sense. 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 45 


TABLE VIII 

Wave Achromatic T. Chromatic T. Difference Ratio 
length (watt X 10-16) (watt 10-16) (watt X 10-16) Yellow=1 
655 ue 626.99 1778 .00 1153.00 01 
616 up 130.28 5620.00 5489.72 .06 

580 uu 27.96 95700.00 95672.04 1.00 

553 um 2.203 855.50 853.30 .009 
522 pp 1.953 1431.40 1429.45 .02 

489 uy 8.11 6430.00 6421.89 .07 

463 wp 15.62 8120.00 8104.38 .08 


The value of the photochromatic interval throughout the spec- 
trum is represented graphically in Figure XXIV. Along the 


106,000 
99000) | 

| } 
gaged | ! 
14,000 
60,000 
50,000 


40,000 


Energy (Watlx 1079) 


38,000 
20,000 


10,000 


Wave Lenath 
655 616 580553 522 409. $63 ee 


Ficure XXIV. Showing value of the photo-chromatic interval (Table VIII) 


vertical coordinate are plotted energy values (watt X 10°"). The 
resulting curve bears to some extent an inverse relation to the 
curve of chromatic sensitivity. Thus, the greatest chromatic 
sensitivity being in the yellow-green, the smallest photochromatic 
interval is in the yellow-green; similarly the largest photochro- 


46 MARGARET M. MONROE 


matic interval is in the yellow, to which there was the least 
chromatic sensitivity. 


D. Comparison with Previous Determinations of the Threshold 
Visibility Curve. In section II is given a summary of various 
investigations that have been made of the relative sensitivity of 
the eye to lights of different wave-lengths. In no case was the 
apparatus and procedure identical with that employed in the 
present study, but it is, however, of interest to make the general 









—Ebert 
—= Langley 
ane Koemg 






—— Green Type 
—-  Yellow-green Type 


Achromatic Sensitivity 








190 100 650 606 560 500 450 +00 rr 


Figure XXV. Comparison of results of threshold visibility curve by various 
observers 


comparison between such curves and the curves presented here. 
Figure XXV shows graphically a comparison of the results of 
Ebert, Konig, Langley, and the two types found in this experi- 
ment. In each case the maximum sensitivity was made equal to 
unity. The curve for Ebert represents the average relative sensi- 
tivity of two observers. It will be remembered that these two 
sets of results were very similar. Konig also gave values for 
only two observers, and these, too, have been averaged here, since 
they show comparatively small differences. The curve for Lang- 
ley is plotted from the results of one observer. The range of 
maximum of the five curves is from A 505 to 553 pu. Koénig’s 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 47 


curve is shifted toward the blue end of the spectrum; Langley’s 
curve, very similar to that of the “ yellow-green type’”’ found 
here, is shifted toward the red end. Ebert’s curve and the curve 
of the “ green type ” fall between these two extremes. So differ- 
ent were the conditions of experimentation in each case that it 
would be futile to attempt to analyze disagreements. Difference 
in source, difference in number (and probably type) of observer, 
difference in methods of energy determination—any one of these 
would seem to be sufficient to account for the variation found. 
Indeed, it is surprising that there should be as much agreement 
as there is. 

It will be noted that the results of Pfluger are not represented 
on this composite graph. The individual and diurnal variations 
in Pfluger’s results were so large as to make averaging for the 
purpose of such a comparison impossible. The position of the 
maximum according to him ranges from A 495 to 524 up, and this 
deviation is found even in the results for a single observer—EI. 
A portion at least of this extreme variation can be explained. In 
the preliminary work of the present study the same difficulty 
obtained. It was soon found, however, that much of this diurnal 
fluctuation could be traced to physiological factors which could 
be controlled—at least in part. A cold, lack of sleep, or general 
fatigue changed the character of the results greatly—not only was 
the absolute sensitivity lessened, but the relative sensitivity was 
altered. Table IX gives the threshold values for the same 
observer taken on different days—once when much fatigued, the 
other when rested. In many colors the difference amounts to 
several hundred per cent. Besides being very insensitive, a 
fatigued observer is erratic, often makes entirely contradictory 
judgments, and is much more subject to troublesome after- 
images and idio-retinal light. Curiously enough, general fatigue 
was found to be more disturbing than eye strain. As has been 
stated in the section on apparatus and procedure, before making 
the final threshold determinations each observer was carefully 
questioned and a few trial settings made as a check on fatigue. 
It is believed that the influence of this factor on the final results 
is negligible. 


48 MARGARET M. MONROE 


TABLE IX 
Yellow- Blue- 
Condition Red Orange Yellow Green Green Green Blue 
Fatigued 2257 .94 676.25 48 .67 4.618 4.558 16.00 19.04 
Rested 305.71 135.42 14.38 1.622 1.718 7.79 13.00 


E. The “ Minimum Visible.’ In the work described in the 
preceding chapters the amount of light required to arouse both 
the chromatic and the achromatic response in different parts of 
the spectrum, the minimum visible for those wave-lengths has 
been measured directly in energy terms for the first time. As 
was pointed out in the historical section, the determinations of 
comparative sensitivity by Ebert, Pfluger, and others were made 
only in relative terms. Absolute values were not assigned to the 
light intensities on which the values were based. Moreover, in 
all cases but one the galvanometer deflections used in compiling 
the ratios were not obtained with the stimulus actually used in 
producing the eye’s reaction. 

Following a different line of development, however, attempts 
have been made to calculate the energy value of the minimum 
visible. The photometric and radiometric data used in making 
these calculations, however, were assembled from different sets of 
observations and experiments. One of the first of such estimates 
is that of Wien (17), who in his dissertation, ‘‘ Ueber die Mes- 
sung von Tonstarken,’ 1888, sought to compare the absolute 
sensitivity of the ear with that of the eye, his data on visibility 
being taken from the observations of the astronomers. He 
assumed that brightness of stars of the sixth magnitude could be 
taken roughly to represent the limit of visibility. By a comparison 
of available photometric and radiometric data he estimated the 
light from those stars to have an approximate value of 4 X 107% 
watt. 

Drude (18), some eleven or twelve years later, calculating also 
from stellar data, obtained a smaller value for the minimum 
visible, 6 X 10°'® watt. He assumed the brightness of a star of 
the sixth magnitude to represent the limit of visibility, a brightness 
which he estimated to be equal to that of the Hefner lamp at 
11,000 meters. Angstrom (19) had determined experimentally 
the energy value of the Hefner unit of illumination (the light 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 49 


emitted in a unit solid angle, the lumen) to be 8.1 & 10* erg/sec. 
The relation of the unit of illumination to the unit of energy or 
power Drude called the mechanical equivalent of light. This 
relation of the photometric to the energy unit has been much 
employed by later investigators in attempts to’arrive at approx- 
imate energy values from measurements made in photometric 
terms. The energy value of the lumen thus, in terms of the 
Hefner standard, would be 8.1 X 10* erg/sec., and the intensity 
of the illumination from a Hefner lamp at 11,000 meters would 
equal 1 X 10° lumens per square meter. Assuming further a 
pupillary opening of 3 mm., Drude calculated: 
Silex 10" 407 ==.6 X10 erg/sec} = 6 X 1071° w. 

This value of Drude has recently been recalculated by Cob- 
lentz (20), who had himself measured the total radiation of a 
Hefner lamp. Coblentz found the light density at 1 meter to be 
7 X 10° g.cal. (or 29 & 10° w) per cm? per second. With this 
value the minimum visible would be: 


29 1Oe 07 
=~ 17 >< 10°'* w. 
. 11000? 


Another calculation of the least radiation visually perceptible 
from astronomical data is that of Ives (21). Ives, too, accepted 
the brightness of stars of the sixth magnitude as representing the 
minimum visible, but recalculated the photometric value of this 
brightness on the basis of later data on the relation of stellar 
magnitude to the candlepower scale. He also used a different 
value of the mechanical equivalent to convert the photometric into 
radiometric terms. Drude had based his determination of the 
mechanical equivalent on the lumen as representing the total radi- 
ation in a solid angle from the Hefner standard. Ives used the 
lumen to represent the photometric unit of radiation from the 
region of the visible spectrum to which the eye is the most sensi- 
_ tive, taken from previous determinations of the relative sensitivity 
of the eye to wave-length. These determinations, however, were 
not made at threshold intensities. If the minimum visible be 
taken to represent the least amount of radiation visually percep- 


50 MARGARET M. MONROE 


tible, of wave-lengths to which the eye is most sensitive, the lumen 
selected by Ives is the more compatible with the purpose of the 
problem. However, the light selected by Drude on which to base 
the ratio of the photometric to the radiometric unit is more nearly 
of the same composition as the light used to produce the eye’s 
reaction. Both methods are in error, but on different points. 
Obviously if the minimum visible is to be determined in absolute 
units for the wave-lengths to which the eye is most sensitive, these 
wave-lengths should be used to determine the threshold of sensa- 
tion and the energy value of the light should be measured directly. 
Using .00159 as the lumen value in watts, Ives made the following 
calculation: 


1 meter-candle = 1 lumen per sq. meter == 0.0001 lumen per ~ 
sq. cm. == .000000159 watt per sq. cm. == 1.59 ergs per sec. per 
sq. cm. 

The meter-candle value of a star of the sixth magnitude had been 
found by Russell to be 0.849 x 10°. Ives, adopting this value, 
obtained 


1.59 X 0.849 X 10° w= 1.35 X 10° erg/sec. 


Assuming a 6 mm. pupillary opening instead of a 3 mm. dasa 
the minimum visible would equal 


38 X 10° erg/sec. = 38 X 107" w 


Russell (22), supplementing the work of Ives, adopted the 
same method of calculation, but used what he thought to be more 
correct figures for the breadth of the pupillary aperture and the 
stellar magnitude of the faintest visible object. He accepted the 
pupillary value of 8.5 mm. proposed by Steavenson and a stellar 
magnitude of 8M.5. The resulting minimum visible was 
77) Kyl Ow 

The above method of calculating by means of stellar data and 
the mechanical equivalent has been varied by the use of an arti- 
ficial star. In this way the photometric determination of the 
threshold can be made experimentally, thus avoiding to some 
extent the uncertainties of stellar observations. Buisson (23) 
measured the distance at which phosphorescent screens of different 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 51 


sizes could just be seen. The brightness of the screen was then 
transmuted into the stellar magnitude scale. Using the same 
values for the mechanical equivalent and pupillary aperture as did 
Ives, he found the minimum visible to be 12.5 K 1077 w. 
Reeves (24) used the transmitted light of a modified Nutting 
sensitometer, the intensity of which could be controlled. The 
source of light was a tungsten lamp. He also measured the 
pupils of his observers instead of taking a standard average value. 
With a visual angle of 1.17 (1 mm. star at 3 meters), the total 
energy entering the eye at threshold intensity was calculated to be 
19.5 & 10°"? w. (mean of three observers). The mechanical 
equivalent was taken equal to .00159 watt per lumen. 

Among the investigation in which some magnitude of star is 
assumed as the limit of visibility, direct energy measurements 
of that source have been made only in one case. Coblentz (20) 
assumed stars of the sixth magnitude to have the least visible 
brightness and measured directly the visible radiation from these 
stars. He states his procedure briefly as follows: ‘‘ The sensi- 
tivity of the eye may be estimated—from direct measurements of 
the heat from stars. The calibration of the radiometer was 1 mm. 
deflection = 34 & 10™** g.-cal. per cm.” per minute = 85.5 & 10°78 
w. per cm.” per second. The sixth magnitude stars gave deflec- 
tions of 0.5 mm. for blue stars to 1.5 mm. for red stars (say, 
1 mm. on an average), depending on their color. From measure- 
ments made on the transmission of stellar radiation through a cell 
of water the radiant luminous efficiency may be 0.2 (0.1 for red 
stars to 0.4 for blue stars). Hence the luminous energy inter- 
cepted by a pupillary opening of 0.07 cm.? is (85.5 & 10°"? x 
ee 0.0/ i 1.2.x, 105° w.” Here, then,, is,.a direct radio- 
metric measurement of the source—there is, however, no corre- 
sponding direct determination of the threshold. 

The above summary of work done by previous investigators 
shows disagreement both as to procedure and as to what shall be 
called “ the least radiation visually perceptible ’’ or “ the minimum 
visible.” Lights differing greatly in composition have been used, 
spectrum and approximate white; and with one exception the 
energy value has been calculated indirectly from measurements 


52 MARGARET M. MONROE 


made on some other source or computed by the use of a mechani- 
cal equivalent which expresses the relation of the photometric to 
the energy unit for some other source than the one used for the 
visual stimulus. And in case of this one exception the limit of 
visibility was taken from astronomical data on the visibility of 
stars, the estimates of which range from the sixth to the eighth 
magnitude, rather than having been experimentally determined. 

It would seem reasonable to assume that the minimum visible 
should mean the least amount of radiation visually perceptible 
of the wave-length or range of wave-lengths to which the eye 
is the most sensitive measured in absolute units. So defined, the 
essential conditions for its determination would be a careful 
search of the spectrum for this wave-length to which the eye is the 
most sensitive, a determination of the limit of visibility with this 
wave-length, and the direct measurement of its energy. In the 
investigations cited above, one and sometimes two, but never all 
of these conditions, have been satisfied. Moreover, in no case, 
whatever the source of light chosen, has the energy been measured 
and an actual determination of the limit of visibility been made 
for the same light." 


1 By a still more rigid interpretation the determination of the minimum 
visible might also involve the satisfying of other and more difficult require- 
ments such as the use of the most favorable time of exposure and size of 
field, the use of the most sensitive part of the retina, etc., all of which 
features would in all probability differ in value both for the wave-length of 
light employed and for the observer. Von Kries and Eyster (25) sought to 
determine the achromatic threshold not only with the wave-length to which 
the eye is most sensitive but also under the most favorable conditions of time 
of exposure, size of field and portion of the retina used. Their selection of 
507 wu as the proper stimulus to use was apparently based not on a determina- 
tion of their own of relative sensitivity but primarily on the fact that 
Trendelenburg had found that this part of the spectrum gave the maximum 
bleaching of the visual purple. According to the Diplicitats theory the wave- 
length that would give the maximum bleaching of the visual purple should be 
theoretically the most correct. Also this wave-length fell within the range 
to which Konig had found the eye to be the most sensitive for a group of 
observers. Their description of procedure leaves one in doubt whether the © 
long and exceedingly difficult systematic survey was made which would be 
needed to determine what part of the retina is the most sensitive to the light 
in question, and what is the optimum size of field and time of exposure. The 
statement is made that the periphery of the retina was used and that several 
sizes of field and times of exposure were used, but no information is given 


-THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 33 


The determination of the minimum visible has been by no 
means the purpose of the present investigation. The energy 
values of the achromatic and chromatic thresholds have been 
determined for seven representative parts of the spectrum. There 
has not been a graded, minute search for the wave-lengths to 
which the eye is the most sensitive. However, two of the points 
used, 522 ym in the green and 553 ym in the yellow-green, fall 
within the range of wave-lengths to which previous investigators 
have found the eye to be the most sensitive at threshold intensi- 
ties for different types of observers. To this extent the require- 
ment that in the determination of the minimum visible the wave- 
length should be used to which the eye is the most sensitive at the 
threshold intensities has been satisfied; the other requirement, 
that the energy measured should be of the light used to produce 
the eye’s reaction, has been fully satisfied. It would seem then 
that our results are entitled to consideration together with those 
purporting to represent the minimum visible, although such a 
determination formed no part of the original purpose and its 
relation to the present work came out only in a study of the 
results obtained. 

A brief summary of the various values that have been obtained 
is given in Table X in order that the values of previous investi- 
gators may be more conveniently compared with those obtained 
here. The apparent closeness of agreement of the results 


as to the approximate meridian or degree of eccentricity of the area of the 
retina stimulated. The source of light was a Hefner lamp the radiations 
from which were reflected from a magnesium oxide surface into a spectro- 
scope. The energy values were calculated from Angstrém’s data on the dis- 
tribution of energy in the light from a Hefner lamp. Von Kries sums his 
conclusions as follows: © 

“1, Fiir eine merkliche Erregung des Sehorgans ist bei Herstellung der 
glinstigsten Bedingungen  hinsichtlich Adaptation, Strahlungsart 
(507 wu) raumlicher und zeitlicher Verhaltnisse eine Energiemenge von 
1,3-2,6 X 10-10 Erg. erforderlich. 

“2. Fiir die Sichtbarkeit dauernd exponierter Objekte ergibt sich bei 
giinstigster Strahlungsart und giistigster raumlicher Anordnung eine 
Energiezufthrung von ca. 5,6 X 10-10 Erg. pro Sekunde.” 

Boswell (26) repeated the experiments of von Kries and Eyster, using an 

amyl acetate lamp as source and the fovea rather than the more sensitive 
peripheral retina. He calculated the minimum visible to be 23.7 X 10-17 watt. 


54 MARGARET M. MONROE 


obtained may not at first glance seem compatible with the rather 
wide disagreement in plan and method of making the determina- 
tions. On this point, however, two comments may be made: 
(a) The percentage disagreement is not small, and (b) a great 
deal of disagreement is doubtless masked by the insensitivity of 
the instrument used to measure the energy values as compared 
with the eye. Coblentz (20), for example, has estimated the eye 
to have 300,000 times the sensitivity of the thermopile. 


TABLE X 

Wien 40.00 X 1 10-l6w Reeves 1.95 X 10-16w 
Drude 6.00 X Von Kries .20 X 
Drude-Coblentz 1.70 “ Boswell 2:0/ Ne 
Ives 380, (Present study) 
Russell O27 ae: Average (553 zu) -27560) 1 
Buisson Ligon ye Average (522up) 2.30X “ 

Smallest value 64 aes 


F. Pathological Cases. The importance of light and color 
sense testing as an aid to diagnosis in pathological conditions of 
the eye is well recognized. There has been, however, little or no 
quantitative study along this line. Although the present appa- 
ratus is, of course, unsuited to clinic work, it was thought of 
interest to determine the achromatic and chromatic thresholds to 
certain wave-lengths in a few typical pathological cases. 

The following patients were sent me by Dr. Luther C. Peter, 
Associate Professor of Ophthalmology, Philadelphia Polyclinic 
and College for Graduates in Medicine, University of Pennsyl- 
vania, whom I wish to thank for his kindness both in cooperating 
in the finding of suitable cases and in furnishing the history of 


each case. 
CASE I 

Mr. Charles P., age fifty-six, cabinet maker. Chief complaint dimness of 
vision. Has been a heavy user of tobacco and alcohol. Vision O D= 20/100, 
O S = 20/150, plus correction = 20/40 partly in the right and 20/70 in the left 
eye, now corrected to 20/40 in each eye. 

Fundus shows atrophic condition of the optic nerves with low grade 
retinitis. Fields are considerably contracted for colors, and both ey spots 
are enlarged for both form and color. 

Diagnosis: Toxic amblyopia (tobacco and alcohol) ; papillo-macular buidie 
is undoubtedly involved in the toxemia. 


Toxic amblyopia—weak vision due to chronic toxemia—may 
be brought on by any toxic agent, but its chief causes are tobacco 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS 55 


and alcohol, either singly or combined. Samelsohn, according to 
Fuchs (27), was the first to discover the anatomical changes that 
take place in the disease. ‘He showed that they were limited 
to the papillo-macular bundles, whose position and course within 
the optic nerve he was thus able to determine. In the course of 
this bundle it is found that the nerve fibers have disappeared and 
nothing but glia tissue is present, while the connective-tissue septa 
lying between the nerve fibers are thickened. Samelsohn regarded 
this as the outcome of an interstitial inflammation of the optic 
nerve, the inflammation affecting the connective tissue portion 
and especially the septa which convey the blood vessels and which 
because of the inflammation become thickened. Others, however, 
think that thickening of the connective tissue is a primary lesion 
of the optic-nerve fibers by the poison, and that if a thickening 
of the connective-tissue septa was found, this was a secondary 
change. Lastly, there are some who believe that even the 
destruction of the nerve fibers is not the primary affection, but 
that, in analogy with the conditions of acute poisoning (by 
quinine, etc.), this consists in a lesion of the ganglion cells in the 
retina and that an ascending atrophy develops in the nerve fibers 
as a secondary affair.’ 

The values of the chromatic and achromatic thresholds obtain2d 
are givenin Table XI. The loss of sensitivity for both color and 
light sense is very great. The loss is not, however, uniform 
throughout the spectrum—there is much irregularity shown from 
wave-length to wave-length. The extremely insensitive reaction 
to blue (right eye) and red (left eye) is particularly noticeable. 


TABLE XI 


ToraL Amount oF Licut ENTERING THE Eye (Mr. Charles P.) 
A. Achromatic Thresholds (watt X 10-16) 


Color Right Left Average (Normal) 
Red (655 wm) 3754.76 1101394 .80 626.99 
Yellow (580 up) 433.73 4888 .48 27 .96 
Green (522 up) 27 .64 34413 .28 1.953 
Blue (463 uu) 61695 .27 2789 .47 15.62 

B. Chromatic Thresholds (watt X 10-12) 
Red (655 wm) doe 297 .640 .178 
Yellow (580 up) 86.530 9.406 9.570 
Green (522 up) 3.820 74.140 143 


Blue (463 uu) 156.440 201.430 .812 


56 MARGARET M. MONROE 


CASE II 


Ruth L., age thirty-eight, single, housework. Bilateral glaucoma when 
twenty-eight years of age. Present condition of the right eye—phthisis bulbi 
and blind. Left eye — vision= 20/15 partly. The left eye was operated on 
twice, first operation about eight years ago, an iridectomy; second operation 
sclero-corneal trephining about eight months ago. Tension normal. 

Fields show some contraction for both form and color. Up and to the 
nasal side of the field is a large angular scotoma extending from and includ- 
ing the blind spot of Mariotte to the periphery including about 1/6 of the 
circumference of the field. The blind area passes in a horizontal line above 
the point of fixation. The macular fibers are not involved by the pathologic 
process in so far as clinical methods can determine. 


Diagnosis: Chronic congestive glaucoma; partial loss of the upper nasal 
field. 


The threshold values for the left eye are given in Table XII. 
Chromatic sensitivity is normal and the achromatic threshold for 
green is normal; the achromatic sensitivity to red and blue, 
however, is very low. The threshold values for yellow were not 
determined, as the eye fatigued rapidly. 


TABLE XII 


ToraL AMouNT oF LiGHT ENTERING THE EYE (Ruth L.) 
A. Achromatic Thresholds (watt X 10-16) 


Color Left Average (Normal) 
Red (655 wu) 2871.54 626.99 
Green (522 up) oroZ 1.953 
Blue (463 um) 70.15 15.62 
B. Chromatic Thresholds (watt X 10-12) 
Red (655 pm) 287 .178 
Green (522 um) .407 .143 
Blue (463 um) .687 .812 
CASE III 


Mr. Wm. A. M., age fifty-one, carpenter. Eyes have been failing for seven 
years. Hypermature cataract in the right eye, incipient cataract in the left. 
In addition the patient is suffering from bilateral sclerosis of the choroid. 
Patient has tubular vision in the left eye, 20/30 partly, central vision, improved 
to 20/20; vision now reduced to 20/40. Light perception only in right eye. 
Form fields are reduced to 10° in the left eye, and red and green varies from 
between 5° and 7°. Light perception and light projection is feebly present 
to the extent of 20°. Wassermann is positive. Clinical diagnosis—sclerosis 
of the choroid and incipient cataract. 


The threshold values obtained are given in Table XIII. Both 
eyes are much below normal, the right more so than the left, as 
would be expected. In this case again the relative sensitivity is 
quite different from that of the normal eye—sensitivity to blue 


ae 


THE ENERGY VALUE OF SPECTRUM WAVE-LENGTHS _ 57 


being disproportionately low. The chromatic responses are most 
erratic, although they approach the normal more nearly than do 
the achromatic. In these, too, is shown the great loss of 


sensitivity to blue. 
TABLE XIII 


TotaL Amount or Licht ENTERING THE Eye (Mr. W. A. M.) 
A. Achromatic Thresholds (watt X 10-16) 


Color Right Left Average (Normal) 
Red (655 up) 164307 .31 35229 .61 626.99 
Yellow (580 up) 496082 .50 5969 .26 27 .96 
Green (522 uu) 61211 .66 7651.46 1.953 
Blue (463 nu) 3759525 .00 1051104.50 15.62 

B. Chromatic Thresholds (watt X 10-12) 
Red (655 um) 12.63 Bs fare .178 
Yellow (580 up) fr ¥ 9.570 
Green (522 up) 9.294 Teist 143 
Blue (463 wu) 463 .84 158.49 .812 


- * Yellow was called colorless even at full intensity. 
+ Green was always called blue, no matter how high the intensity. 


CASE IV 

John H., age sixteen. Secondary atrophy of the optic nerve following 
injury to the orbit by an automobile. Vision O D = 20/20 partly, O S= 20/70. 
This has since been reduced to 20/500 in the left eye. The ophthalmoscope 
shows a white nerve head with much contraction of arteries and veins. The 
form field is reduced to about 10° and green is visible 3° beyond the point of 
fixation (by clinical methods of study). 

Clinical diagnosis: Progressive secondary optic atrophy (traumatic). 

The energy values for the achromatic thresholds cannot be 
given—both eyes fatigued very rapidly, and as the first rough 
settings showed that the threshold values would fall well within 
the normal range, the more accurate determination was not under- 
taken. The chromatic thresholds values for the injured eye are 
given in Table XIV. Sensitivity to green and blue is normal; 


sensitivity to red is greatly reduced. 


TABLE XIV 


ToraL Amount or Licht ENTERING THE Eve (John H.) 
Chromatic Thresholds (watt * 10-12) 


Color Left Average (Normal) 
Red (655 wu) 1700.00 .178 
Green (522 up) .653 143 
Blue (463 pp) .106 .812 
CASE V 


John B. H., age twenty-three, student. Convergent unilateral squint since 
early childhood. Vision without glasses: O D= 20/500, O S= 20/300. Vision 


58 MARGARET M. MONROE 


with glasses*i:0O D450», S°O \ 25) Ciax90—=20/300,5.0.5==4008 S OF50 
Cax 90—=20/12. Right eye shows convergent squint of 20°. Peripheral 
vision good; macular vision in the squinting eye amblyopic. 

Diagnosis: Amblyopic ex anopsia as the result of the squint. 


Amblyopia ex anopsia—defective vision attributed to lack of 
use—“ may occur on account of obstruction to the rays of light : 
falling upon the retina—e.g., congenital corneal opacities, con- 
genital cataract, and impervious persisting pupillary membrane; 
or in an eye which from early infancy has squinted, and has, 
therefore, not been concerned in the visual act.’ Parker (28) 
gives the following description of the process: ‘‘ Because of 
defect in the fusion faculty, aided, perhaps, by hypermetropia 
(far-sightedness) and possibly from debility from disease, one 
eye shows an occasional transitory squint. This at first produces 
a diplopia (double images) from the fact that the two images do 
not fall on corresponding spots of the retinae. The eyes right 
themselves by muscular effort to parallelism to avoid this diplopia, 
but this power is soon lost, and the image of the squinting eye. 
is suppressed, at first much as we would suppress the images 
falling on the left retina when we are looking through a micro- 
scope with the right eye; finally the squint becomes constant, 
diplopia no longer is noticed, and the retina of the squinting eye 
ceases to functionate. This condition is properly called amblyopia 
ex anopsia.” 

As is shown in Table XV, both the light and color sense of the 
defective eye were found to be normal. This is what was 
expected by Dr. Peter, who believed it to be a matter of defective 
spatial perception. 

TABLE XV. 


Tota Amount or LicHt ENTERING THE Eye (John B. H.) 
Achromatic Thresholds (watt X 10-16) 


Color Right Average (Normal) 
Red (655 pu) 560.00 626.99 
Yellow (580 uu) 26.46 27 .96 
Green (522 um) 3.811 1.953 
Blue (463 uu) 16.48 15.62 

Chromatic Thresholds (watt X 10-12) 
Red (655 up) 17 .178 
Yellow (580 uu) 4.68 9.570 
Green (522 uu) 1.219 143 


Blue (463 up) 2.102 .812 


10. 


11. 


12 


13. 


14. 


15. 
16. 


WWE 


BIBLIOGRAPHY 


Historical Summary 


. Epert, H. Ueber den Einfluss der Schwellenwerthe der Lichtempfindung 


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59 


60 


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MARGARET M. MONROE 


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Pathological Cases 
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1910. 


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